How to wire a new thermostat

Discussion in 'HVAC Heating & Cooling' started by Blumengarten, Nov 3, 2013.

  1. Blumengarten

    Blumengarten New Member

    Messages:
    56
    Location:
    Pennsylvania
    When I turned my furnace (we have radiators) on for the winter, the furnace wouldn't turn off .. it was over 90° in here! I asked a handy friend what he thought it would be, he said first to replace the thermostat, if it's not that it's probably a thermocouple, but try the thermostat first. So I bought a new Honeywell CT87K (for heating only, we don't have air conditioning).

    There were only two wires connected to the old Honeywell thermostat, a red won connected to a screw in the plate, the blue one connected to a screw on the thermostat. The blue, yellow, green, and white wires were folded back into the hole.

    The directions for the new thermostat said to label the wires, but I didn't because there were no letters or any indication of anything on the old thermostat. It says to connect the R wire to the R on the thermostat and W to the W -- there are directions also for a 3-wire hot-water heat-only system, but since the old thermostat only used two wires, I think I should use the two wires. I'm not an electrician, just a mom, but I do know that the colors are just aids, and it doesn't matter which color wire you use just as long as you are consistent at each end, so I shouldn't be concerned that whoever installed the thermostat used a blue wire instead of white. But would you just put the blue wire in the W slot?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,018
    Location:
    New England
    Open the cover on the boiler (a furnace is a hot air device - a boiler is a hot water or steam device) and see which color lead is on the R position, and put that on the R terminal of the new thermostat. The other wire should go on the W terminal of the thermostat. Some thermostats won't care if they are switched, it's possible some do, but it's better to not guess. Essentially all the thermostat is doing is connect those two wires together when it needs to start the heating system, and breaks the connection when it's done...just like a light switch.

    If I were going to guess, I'd use the red wire to the R terminal, and the other one to the W terminal. Some of the thermostats also have internal switches or controls to tell the thing what type of heat you have...and, some allow you to adjust the tolerance on the temperature control. With a boiler, if you have a choice, you probably don't want to control it too precisely - it will potentially make the boiler run shorter cycles, which isn't as efficient.
  3. Blumengarten

    Blumengarten New Member

    Messages:
    56
    Location:
    Pennsylvania
    Thanks very much, I appreciate your help!!
  4. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,175
    Location:
    Maine
    I doubt the thermostat is the problem.
  5. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,150
    Location:
    Houston, TX

    I agree, unless it is located in a bad location.

    I don't think it would be the thermocouple either.

    The Fan limit switch may need to be looked at, If it is a furnace.
    Last edited: Nov 4, 2013
  6. Reach4

    Reach4 Active Member

    Messages:
    2,256
    Location:
    IL
    I would look at the furnace end of the wires. There you may find the letters discussed in the thermostat instructions. In particular you might have a G wire to allow turning on the fan, and a C wire that could be used if you want to replace that new thermostat with a WiFi thermostat some day. Turning on the fan can be useful to even out heat. A $100 WiFi thermostat is good for letting you control or monitor temperature remotely as well as being a handy way to set up a schedule.

    Note when you look into the furnace where you can see the control, there is probably an interlock switch that will prevent the furnace from working while that cover is open. So don't think you broke something just because the furnace does not operate with the cover open.

    You can use different colors. Did the thermostat come with labels? If not, you can make your own tiny labels for wires that will help you or somebody else in the future.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,018
    Location:
    New England
    FWIW, it's actually a boiler (the OP said furnace, but it has radiators).

    If the thermostat is actually working, and it has something like zone valves, a zone valve may be stuck open, or it was manually opened, and cannot close. If it only relies on the circulator, I guess it's possible the relay controlling it is welded shut. Usually, in my limited experience, a relay coil opens up or the contacts get burned, and it fails to make the connection, but I have seen one weld itself closed...rare, though.
  8. Blumengarten

    Blumengarten New Member

    Messages:
    56
    Location:
    Pennsylvania
    My inclination like yours was that it's not the thermostat, but it's the easiest and cheapest thing to fix. I haven't needed to turn the furnace on during the day, so we'll see how it goes tonight. After the thermostat, I'm not willing to do any work on the furnace, of which I know absolutely nothing! Thanks for your comments though, it will give me some knowledge when the furnace man comes (and the new thermostat is prettier than the old, gold one!)

    Thanks,
    Blumengarten
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,018
    Location:
    New England
    If you actually have a boiler (pumping hot water around to radiators), when you call for service, tell them that. While many service people can handle both hot air furnaces and boilers, some specialize, and, you don't want to pay for a furnace guy and find out he knows little about boilers!
  10. Blumengarten

    Blumengarten New Member

    Messages:
    56
    Location:
    Pennsylvania
    That's a great point, I hadn't thought about that. Thanks for mentioning it!
  11. Blumengarten

    Blumengarten New Member

    Messages:
    56
    Location:
    Pennsylvania
    It finally got cold again, so I turned the furnace back on yesterday, and it turned itself off, so it was the thermostat after all! Hooray, I'm glad I saved a call to the furnace man. Thanks gentlemen for your help, I appreciate it! Since my father's dead and my husband's not handy, it sure is good to know someplace reliable to turn to for help.
  12. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,150
    Location:
    Houston, TX

    Great that you got it going.

    Maybe you were just getting Hot Flashes. lol


    Enjoy.
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